The purpose of this project is to characterize the nucleoprotein of the free-living thermophilic mycoplasma, Thermoplasma acidophilum. This organism is particularly unusual among prokaryotic cells in that there is a protein tightly bound to its DNA that resembles the histones of eukaryotes. One function of this protein appears to be in protecting the DNA from thermal denaturation. Studies on this protein might contribute to our understanding of the evolutionary origin and physiological functions of eukaryotic histones. We have evidence that the nucleoprotein structure in T. acidophilum is based upon repeating subunits, similar to that of eukaryotic chromatin. The DNA appears to be coiled into a loop aroung a core of protein. However, the loop is only about 40 base pairs long, and therefore is much smaller in size than in eukaryotic nucleosomes. We have yet to determine how many proten molecules comprise the core, and whether the DNA is partially denatured in order to twist it into a loop of this short circumference. Other possible physiological functions of the histone-like protein will also be tested, including whether the DNA is protected from depurination, and also, whether the osmotic effect of the DNA is decreased by its association with this protein. In addition, the amino acid sequence of the protein is being determined (in collaboration with Dr. Robert DeLange of U.C.L.A.), and a similar protein in another prokaryotic organism, Sulfolobus acidocaldarius, is being characterized.